1. Field of Invention
The present invention relates to a serial tandem image forming system and a method of controlling the same.
2. Description of Related Art
In recent years, a serial tandem image forming system is known which includes a plurality of image forming apparatuses connected in series. For example, in the case of an image forming system consisting of two serially connected image forming apparatuses, the upstream side image forming apparatus (upstream apparatus) forms an image on one side of a sheet, and then the downstream side image forming apparatus (downstream apparatus) forms an image on the other side of the sheet. This improves the productivity in comparison with double-side printing performed by an individual image forming apparatus (for example, refer to Japanese Patent Published Application No. 2012-91357).
The image forming apparatus may for example be an electrophotographic image forming unit. Such an electrophotographic image forming unit performs a series of processes which include forming a toner image on an image bearing member, transferring the image to a sheet, and then fixing the image to the sheet. In this case, since there may be different operating states suitable for processing different sheet types respectively, the operating state of the image forming apparatus is switched in accordance with the sheet type, for example, between a cardboard and a thin sheet. The operating state can be designated by a fixing temperature, a nipping pressure at a transfer unit and/or a fixing unit, and so forth.
A print job may include not only a process of sequentially printing images on a plurality of the same type sheets, but also processes of printing images on a plurality of different type sheets which are switched during printing. In the former case, the print job can be performed with no need for switching the operating state of the image forming apparatus. On the other hand, in the latter case, the print job is performed by switching the operating state of the image forming apparatus with a timing when switching the sheet type between a preceding sheet and the subsequent sheet following the preceding sheet. A sheet cannot be conveyed while switching the operating state, and therefore the subsequent sheet is temporarily stopped at a registration unit from which each sheet is sent out to an image formation process (a transfer process with a transfer unit and a fixing process with a fixing unit). The subsequent sheet is conveyed again after finishing the switching of the operating state.
Namely, in the case of the serial tandem image forming system, the subsequent sheet is standing by at the registration unit of an upstream apparatus until the switching of the operating state is finished. In this case, since the sheet conveyance distance of the serial tandem image forming system is longer than that of a single image forming apparatus, the productivity may be reduced due to the distance between the subsequent sheet and the preceding sheet which becomes wider when halting and resuming the conveyance of the subsequent sheet.
It is conceived to have the subsequent sheet stand by after further conveying the subsequent sheet to an advanced position in the downstream side of the registration unit in the upstream apparatus to shorten the conveyance distance after the resumption of sheet conveyance. However, in this case, the subsequent sheet may stop in the middle of the image formation process, for example, in a transfer unit or a fixing unit. The image quality may thereby be degraded depending upon the stopping position. Particularly, while an image is to be formed also by the downstream apparatus on the same sheet, it is undesirable that such a situation occurs in the upstream apparatus.
The present invention has been made in order to solve the problems as described above. It is an object of the present invention therefore to inhibit both the reduction of the degradation of image quality and the reduction of the productivity.